Conventional air conditioning systems utilize a compressor to compress refrigerant gas, a condenser to remove some of the heat of the refrigerant gas and to condense the refrigerant to a high pressure liquid, and an evaporator that uses the refrigerant to cool a supply of water that in turn cools the space that is being air conditioned. Air conditioners utilize a number of different designs of compressors depending on the application that the air conditioning system is being used for. Although applicable to all lubricated air conditioning compressors, the present invention is described as applied to a screw compressor.
A screw compressor is a positive-displacement compressor that uses a first rotor driving a second rotor (termed the driven rotor and the slave rotor) to provide the compression cycle. The two rotors each have cooperating helical lobes that are interleaved with each other. Since the driven rotor drives the slave rotor by means of the interleaved lobes of the two rotors, the two rotors are necessarily counter-rotating. The design uses injected oil to cool the compressed refrigerant gas, to seal the volume created between the rotors in which the refrigerant is compressed and to lubricate the rotor bearings. A comparatively large oil capacity, as much as a flow rate of ten gallons (37.854 liters) per minute in some applications, is required to perform these functions. This large quantity of oil is injected directly into the lobe area as the refrigerant is being compressed. The oil and the refrigerant in this way become thoroughly intermixed.
A typical screw compressor system consists of the compressor, motor, oil/gas separator and reservoir. The screw compressor may also include an oil cooler with a filter. The motor drives the driven rotor either directly or through a gearset. Direct drive is preferable to avoid the mechanical losses that result from the gearset. Rotational speeds of the compressor are on the order of 3600 RPM. Since the oil is injected directly into the rotor area of the compressor, the oil and the refrigerant gas mix. A single output line transports the mixed compressed refrigerant and hot oil to an oil/gas separator. The oil separator separates the oil and the refrigerant and stores the oil temporarily in the reservoir prior to sending the oil to the oil cooler. The oil cooler cools the oil to a temperature at which the oil has good lubricating properties and can again cool the compressor. The cooled oil is then pumped back to the compressor where the lubricating and cooling cycle of the oil begins anew.
The compression process starts with the rotors interleaved at the inlet port of the compressor. As the rotors turn, the lobes are separated, causing a reduction in pressure, drawing the refrigerant in through the inlet port. The refrigerant fills the volume defined between the lobe of the driven rotor and the lobe of the slave rotor. The intake cycle is completed when the lobes have turned far enough to be sealed off from the inlet port. As the lobes continue to turn, the volume of the space defined by the lobes between the meshing point of the rotors is continuously decreased. By continuously decreasing the volume as the helical rotors rotate, the refrigerant that was drawn through the inlet into the volume between the lobes is compressed. Ultimately, the interleaved lobes open to the discharge port, allowing the compressed refrigerant and the entrained oil to flow out of the compressor through a common line to the oil/gas separator.
The ratio of the volume of gas trapped after the intake cycle to the volume of gas trapped just before the lobe opens to the discharge port is known as the built-in volume ratio. With the injected oil performing a majority of the cooling and with very low pressure differential across each lobe, the screw compressor can reach built-in volume ratios as high as 20:1 when operating at full capacity. Additionally, throttling controls are typically included in order to permit the screw compressor to operate over a wide range of capacities depending on the amount air conditioning cooling required at any given time. A typical throttle comprises a sliding valve that slides in the valley formed between the interleaved rotors. The valve discharges the compressed refrigerant from the compressor before the refrigerant has undergone the full extent of compression possible.
As noted above, the refrigerant gas temperature rises dramatically due to the heat generated by the refrigerant gas as the gas undergoes compression. The injected oil cools the refrigerant gas and the compressor. By keeping the compressor relatively cool, the oil enables the compressor to attain high volume ratios, thereby increasing the efficiency of the screw compressor. The discharge temperature of a screw compressor seldom exceeds 200.degree. F. (93.degree. C.), with the normal temperatures running around 160.degree. F. to 180.degree. F. (71.degree. C. to 82.degree. C.). The oil flow through the compressor removes up to forty percent of the heat generated by the compression of the refrigerant gas.
The oil also forms a film between the two rotors to allow the drive rotor to turn the driven rotor without metal-to-metal contact. Effectively, a thin film of oil resides between the lobes of the two rotors and transmits the driving force of the driven rotor to the slave rotor without the driven rotor actually coming in contact with the slave rotor. This greatly increases the life cycle of the compressor by reducing wear.
It is imperative that the oil injected directly into the refrigerant gas stream during the compression cycle be separated from the refrigerant after compression is complete. Oil/gas separators are normally designed to accomplish such separation by mechanical means that exploit the fact that the liquid oil is heavier than the gaseous refrigerant. The oil/gas separator also temporarily stores the oil prior to conveying the oil to the oil cooler before sending the oil back the compressor. A reservoir is formed at the bottom of the oil separator to hold the oil.
A major portion of the heat of compression generated in the compression of the refrigerant gas is retained by the liquid oil. The oil must accordingly be cooled prior to recirculation to the compressor. The cooling of the oil also improves the lubrication properties of the oil and extends the useful life of the oil. The oil cooler is a heat exchanger in which the heat of the hot oil is rejected to a cooling medium, such as water, ethylene glycol, or air. The cooling capacity of the oil cooler is matched to the rest of the screw compressor so that the oil is returned to the compressor at a desired temperature.
Loss of the oil charge in the compressor is a serious problem since, as indicated, so much of the operation of the compressor is dependent on the continued flow of oil. An operating screw compressor will effectively destroy itself within several minutes of operation after an oil loss occurrence. It is, therefore, critical to be able to detect such a loss and to shut the compressor down before the loss causes catastrophic damage to the compressor. For purposes of this invention, an oil loss is defined as at least one of the following conditions: (1) loss of oil due to leakage in the system, (2) inadequate oil flow due to a restriction in the lubrication lines caused, for example, by a blockage, clogged filter or a malfunctioning valve, and (3) inadequate oil cooling caused by plugged condenser coil fan malfunction or the like.
A number of methods have been put forward in order to ensure that the compressor has an adequate oil charge. U.S. Pat. No. 3,232,519 proposes utilizing a fairly large number of sensors in the compressor to detect abnormal temperatures and abnormal temperature differentials. The sensed values are compared with a series of setpoints that are fixed in the control system. Such indications are then assumed to be indicative of a particular problem in the oil delivery system. Unless a system always operates under exactly the same conditions, however, the setpoints must necessarily be a compromise to capture as wide a range of operating conditions as possible without being so wide as to be meaningless for the extreme operating conditions. A more useful standard would be one that is variable with the operating conditions and is therefore meaningful for all operating conditions.
U.S. Pat. No. 4,583,919 proposes utilizing a sensor of the temperature of the oil at the inlet to the compressor to determine if additional oil is needed. The temperature of the oil is compared to a setpoint and when that setpoint is exceeded, a second oil line is opened to the compressor.
U.S. Pat. No. 5,062,277 is concerned with oil loss. The '277 patent discloses the use of an oil heater to detect oil loss by sensing the temperature of the heater. The heater normally is submerged in the oil at a selected level in the oil tank. The heater uses the oil that is in the tank as a heat sink. If the temperature of the heater is sensed to have risen unduly, it is assumed that the oil has dropped below the level of the heater, since the heat sink is no longer available to draw the heat off of the heater.
It would be a decided advantage to have a reliable means of early detection of loss of oil charge in a screw compressor. The detector should permit timely shutdown of the screw compressor prior to the occurrence of any internal damage thereof resulting from the loss of oil. The method of detection must be effective over all operating conditions of the screw compressor and therefore should not be limited to comparison to a setpoint.
When the screw compressor package suffers a loss of oil charge, the reservoir of the oil/gas separator and the oil cooler fill with refrigerant. This refrigerant then undergoes the cooling in the oil cooler that is meant for the oil. It is this characteristic of the screw compressor package and the fact that the oil and the refrigerant have different heat transfer coefficients that make possible the oil loss detection method of the present invention.